Multi-layered ceramic (MLC) packages, specifically single chip modules (SCM) and multi-chip modules (MCM), are used in a number of applications. Such modules generally consist of a substrate and a cap that must be bonded together to a specified dimensional tolerance, usually using adhesives that are heat-cured. Industry competitiveness demands low-cost, high-volume, high-yield assembly of such modules.
Stamped fixtures, typically of stainless steel, such as "Auer Boats" manufactured by AUER Precision Company, Inc. of Mesa, Ariz., are prevalent in the industry as fixtures used for such assembly. Such fixtures incur high initial tooling costs, however, for even minor variations in MLC package design, and are not compatible with conveyor equipment designed to work with a tool set complying with the Joint Electronic Device Engineering Council (JEDEC) Standards. Use of typical stamped fixtures requires additional handling which is labor intensive and also results in yield loss due to handling defects. In addition, loose cap-to-substrate alignment tolerances decrease the usable substrate area.
The configuration of a typical stamped stainless steel fixture of the prior art for assembly of an MLC package is shown in FIG. 1. A typical MLC package consists of a substrate 100 and cap 102, the substrate having mounted upon it an integrated circuit chip 104 and having pins 106 extending from the bottom of the substrate.
To assemble the MLC package, the substrate containing the chip is set in a base 110 aligned by guides 111. An alignment plate 112 is aligned on the substrate using alignment pins 114 attached to the alignment plate, each pin comprising a spacer portion 116 and a pin portion 118 adapted to fit in hole 119 in base 110. Substrate 100 and chip 104 are typically prepared with chip underfill applied around and wicking under chip 104, thermally conductive paste applied on top of chip 104, and seal adhesive placed on the surface of substrate 100 where cap 102 will contact the substrate. The underfill protects the chip-substrate interface and prevents oxidation of the solder used to attach the chip pins to the substrate. The thermally conductive paste creates a conductive pathway from the top of the chip to the cap, so that heat may be dissipated away from the chip through the cap. Finally, the seal adhesive bonds around the perimeter of the cap to seal the area inside the cap to protect it from oxidation. Cap 102 is then placed on top of the substrate so prepared.
Pressure is then applied to press the substrate against the cap, using clip 120. Clip 120 consists of a bridge 122 having tabs 124 punched therethrough, and prongs 126 attached at either end of bridge 122. Each prong 126 has an upper stop tab 128, a lower stop tab 130, and an angled end 132. Tabs 124 are spaced to hold the ends of leaf spring 134 therebetween. The compression force, usually 2 to 10 pounds, imparted by the spring serves to "squish" the paste layer on top of the chip to conform it to the space between the chip and the cap, thus assuring a good conductive connection and cap seal.
The clip 120 is inserted manually by squeezing the prongs 126 slightly toward one another and inserting them through alignment plate holes 136 and baseplate holes 138 thus compressing spring 134. Once the prongs 126 have completely penetrated holes 138, the prongs are allowed to spring back away from one another, and the lower tabs 130 hold the prongs into place to prevent the force of compressed spring 134 from retracting the clip. Upper tabs 128 prevent the prongs from being inserted too far into holes 138 such that too much compressive force would be imparted on cap 102. The MLC package so assembled is then put into an oven to heat cure the seal adhesive. Cured modules then must be removed manually from the fixture.
A typical stamped stainless steel fixture of the prior art might accommodate anywhere from one to ten such MLC packages, but typically no more than five on a single base with a single corresponding alignment plate. One clip for each MLC package on the base must be inserted and removed manually, however, for each MLC package. Manual removal of the clip requires compressing prongs 126 toward each other a sufficient amount to clear tabs 128 and 130 through holes 138 and 136. This operation usually requires handling the clip at ends 132, sometimes causing finger damage to pins 106 if mishandled.
Another difficulty associated with the stamped fixtures of the prior art is that they cannot be stacked on top of one another. Because the oven curing step may often take three to four hours, the ability to maximize the number of fixtures in a single oven can dramatically increase throughput without requiring investment in additional ovens.
In addition, because the fixtures of the prior art are manufactured by a stamped metal process having inherently loose tolerances, the alignment between the base and the substrate may not meet the tight tolerances of customer specifications. As the industry strives to ever increase the amount of performance per size of unit, more of the substrate closer to the edges is typically used for functional design, and thus close tolerances of cap-to-substrate alignment have become more critical.
Finally, such fixtures must be re-tooled to accommodate even minor variations in MLC package width, length, or height, a serious drawback considering the common height variation in MLC packaging. In addition, a fixture is needed that uses industry standard JEDEC trays.
It is an object of the present invention, therefore, to provide a fixture flexible enough to handle the variations in package dimensions from product line to product line, capable of high-volume production with high yield conforming to precise dimensional tolerances, and strong enough to withstand constant manufacturing use.